SO39i Inverter and Microwave Suggestions

[Erik]

I have a 2007 Sun Odyssey 39i. I would like to install a microwave in the cabinet area above the stove, and would like to also install an inverter to go with it.  I have 3 house batteries totaling 200Ah (maybe a little more)...they're lead acid, nothing fancy. I have 2 solar panels totaling 270 watts which helps keep the batteries topped off. No generator, and I'm usually not plugged in to shore power.  I'm prepared to run the engine if I need to use the microwave (40amp alternator).  I'm looking for recommendations for a microwave oven that fits (likely 800 watts to keep the draw low), and also for the right inverter (I'm thinking 1500 watts will be sufficient) and guidance on how to wire it (i.e. so that it feeds the outlets and the microwave not the hot water heater etc.)

[zaphod]

Before you start shopping for microwaves, you should be taking a closer look at your electrical system. there is no way your 200AH "nothing special" flooded lead acid batteries are up to the task of handling a 1000w inverter load, nor is your 40a alternator! (1000w/12v = 83amps!)

Being able to run things like microwaves on your battery bank is the reward for building a beefy battery bank with high quality components and upgraded charging systems. Think over 400ah in high end batteries, 60a or more battery charger.(the Cristec charger my boat, and probably yours came with is not very good.) On the bright side, you could go with an inverter/charger combo. Xantrez makes a 1000w 80a inverter/charger that I am contemplating.

And don't forget the 100a alternator and external regulator! Even 400ah will drain pretty fast if you are running 1000w loads for minutes at a time. You are going to want to put it all back reasonably fast. Higher end batteries need different charge profiles and smarter operation than the stock alternators are capable of.

I personally think microwaves are best left at home, but if it is a priority for you, be prepared to go down a deep and expensive rabbit hole! You could also install one for shorepower use only!

[Charlie-Bravo]

If your current battery set up is in reasonable shape and does everything you need .... except the microwave,  you might find leaving everything alone and buying a generator an option, a reasonably quiet honda of a good wattage output to vastly exceed the microwave need might serve, and could power other things as well, more petrol on board , but you probably have some for the outboard anyway.
Not a fully sorted solution I know, but as with all things it's down to choices, money ,time, and how much you really want a microwave.
Could be much cheaper than the new batteries , alternator and 'things' that a beefy battery bank requires.

I will stay with the gas oven I think, but each to their own.
CB

[Erik]

West Marine's Chuck Hawley suggests a rule of thumb that for every 1000W put out by the inverter to have 200Ah of battery bank (https://www.youtube.com/watch?list=PLDCA39C2CAA05E8BF&v=fkwfydHVJMk&feature=emb_logo).  I have 240 Ah in my battery bank (new batteries), which suggests it can support a 1200W inverter.

Let's do some math to see if this makes sense:

If I run a 700W microwave oven, it will draw 700W / 12v = 58 amps * 1.1 for inverter inefficiency = 64 amps.  If I run it for 5 minutes, that would be 64 amps * (5 / 60) = 5.3 Ah.  It should be no problem for a 240 Ah bank to support 5.3 Ah.

To recharge I've got solar and/or the engine.  Either is capable of replacing that charge.

I don't live on the boat, and I do use the stove most of the time so I would probably use the microwave for 5 or 10 minutes per day maximum, and not every day.

I can buy an inverter for $500 and a microwave for $200, and I can install it myself.  A genset would be thousands and someone else would have to install it, another thru-hull to worry about, and then there's the maintenance, winterizing, etc.

As for an inverter charger, my charger is brand new (ProMariner 1240p which replaced the original Cristec after a lightning strike) so I'd rather stick to a dedicated inverter (which is what Jeanneau prefers when an inverter is installed at the factory according to the dealer).

The alternator is also brand new thanks to the lightning strike, so I'd rather not replace it, and again I think that 40amps is sufficient to recharge batteries after a few minutes of microwave use every now and then.

What am I missing here...?

[zaphod]

Aug 2, 2020 19:12:20 GMT Erik said:

West Marine's Chuck Hawley suggests a rule of thumb that for every 1000W put out by the inverter to have 200Ah of battery bank (https://www.youtube.com/watch?list=PLDCA39C2CAA05E8BF&v=fkwfydHVJMk&feature=emb_logo).  I have 240 Ah in my battery bank (new batteries), which suggests it can support a 1200W inverter.

Let's do some math to see if this makes sense:

If I run a 700W microwave oven, it will draw 700W / 12v = 58 amps * 1.1 for inverter inefficiency = 64 amps.  If I run it for 5 minutes, that would be 64 amps * (5 / 60) = 5.3 Ah.  It should be no problem for a 240 Ah bank to support 5.3 Ah.

To recharge I've got solar and/or the engine.  Either is capable of replacing that charge.

I don't live on the boat, and I do use the stove most of the time so I would probably use the microwave for 5 or 10 minutes per day maximum, and not every day.

I can buy an inverter for $500 and a microwave for $200, and I can install it myself.  A genset would be thousands and someone else would have to install it, another thru-hull to worry about, and then there's the maintenance, winterizing, etc.

As for an inverter charger, my charger is brand new (ProMariner 1240p which replaced the original Cristec after a lightning strike) so I'd rather stick to a dedicated inverter (which is what Jeanneau prefers when an inverter is installed at the factory according to the dealer).

The alternator is also brand new thanks to the lightning strike, so I'd rather not replace it, and again I think that 40amps is sufficient to recharge batteries after a few minutes of microwave use every now and then.

What am I missing here...?

Keep in mind that with flooded lead acid batteries you can only draw them down to 50%, so your 240ah bank will only give you 120ah of useable power, and that assumes your batteries are in perfect condition.

I suppose if you are only using the microwave 5 minutes at a time you might be ok. I'm not sure how much cooking you can do with 5 minutes in a 700w microwave.

If you intend to be off grid for any length of time you will find that you are running your engine a lot to keep the batteries charged. Your alternator may be rated for 40a but that doesnt mean it always puts out that much.

I don't know why you want to be "that guy" in the anchorage always running his engine just so you can nuke your food in a microwave, but by all means give it a shot!

[zaphod]

This morning, I am sitting in a peaceful Anchorage enjoying the glassy calm conditions, listening to the wildlife....then a boat near me fired up his engine to charge his batteries.😡

Don't be that guy!

[Charlie-Bravo]

Go easy, .....  each to their own, ..  the motor won't run forever, at least your at anchor in a nice place, we are hunkered down here with rain rather heavy, enjoy.
CB

[alenka]

Hi Erik,

I cannot remember the last time we used our microwave to cook food. Perhaps I come from that generation that didn't buy into microwaves.  On our boat it is a good place to store the electric toaster however!

The thing to look for with inverters is both the peak and sustainable output it can handle. All too often it might say 1500watt inverter on the box but that's only a peak value that it can handle for a few seconds and the sustained output might be only 800watts. As I am sure you are already aware there is nearly always a large spike when starting a new appliance before it settles down to its rated value.  

The same overselling is often applied to cheap generators.

Buy cheap... buy twice... As some would say, so maybe look for something with 30% headroom on both ratings at the very least.

Our inverter is used mainly to power my wife hairdryer which she uses when the sun is high in the sky and the solar panels have time to replace the energy. With careful planning we only very rarely get caught out with low batteries - but we are blessed with an abundance of sunshine on most days. 

[Erik]

I appreciate all of the replies, but I was really hoping to get some technical information, and not just rants about why I can't or shouldn't install a microwave oven on my boat.  If there are any engineers or engineering minded people out there that have done this, I would appreciate some specific recommendations....!  Thanks.

[Tafika II]

Hi Erik!
We have a 1000w Microwave on board that we run off 30A shore power, 3KVA genset, engine running, or 1800w inverter. The house battery bank is 630Ah, with two solar panels totaling 340W and wind generator. We have a Balmar 120amp alternator with a controller. Rarely have voltage draw problems but the microwave, toaster, and hairdryer do draw down the batteries more significantly. I hope this helps!

[alenka]

Erik,

Youtube usually has a whole load of 'how to' videos but installing a microwave on a boat is a bit sparse as a subject.  However, there are plenty on fitting one into a motorhome and these are usually less well equipped with batteries.



This is short and sweet and talks about an extra dedicated leisure battery running an 800watt microwave through a 1500watt inverter. The battery lasts about 30 mins.

The video touches on camper van charger problems. But if I was adding an extra battery for this purpose I would use a simple battery to battery charger.



Good Luck. 

[rene460]

Hi Erik,

I think you have addressed the main issues in describing your plan, and I can think of many good reasons to have a microwave on board, but to address your basic question of “what are you are missing“, I have a few thoughts that might be worth considering.

I know how hard it is to cover everything you have thought about in a brief post so you have probably covered the main things, but it might be worth checking the effects of some of the assumptions, to see what would result if they turn out overly optimistic.

First, I suspect you you might use the microwave considerably more than you have estimated.  We use a 1200 watt unit at home and have a good idea of how long things take.  We don’t have one on our boat, I suggested including it, but my wife put more priority on the cupboard space on our small boat.  But when we are in the marina, we have access to the microwave in the berth holders facilities, and use it quite regularly.  It is about a 600 watt unit, and we find it quite slow.  Most things take around twice as long as the one at home.  So I would suggest checking how the calculations look with say double your present estimate for your intended uses.

Second, with the batteries, I am with the comments made earlier about only draining them to 50% capacity if you want reasonable battery life.  In addition most solar and alternator charging Systems only achieve around 90% maximum charge, so you only have about 40% of the rated capacity available.  In addition, there are losses in both charging and discharging the batteries so it is necessary to assume that not all the energy used in charging is available to retrieve later.  I believe that something around 80% might be reasonable, so you may need to charge longer than you have first assumed to replace what you use.  In the years I have been a member of this forum, I can remember many more comments about needing more capacity and 200 amp.hr is only the first step above the basic single house battery supplied as standard equipment.  Even though not living aboard, you no doubt use other devices while you are on board, instruments, radio, fridge, and so on, and most of us find we are living on the edge of sufficient capacity.  So the apparently small consumption may be a big proportion of the “excess capacity” you may have available before you start on this project.

Third on the charging capacity.  Remember the solar cells are rated based on 1000 watts per square meter from the sun, a reasonably bright cloud free sunny day, with the panel facing directly at the sun.  In the tropics you get a bit more.  In real life, most of our panels rarely face directly at the sun, and for much of the day they produce much less that the rated output.  And most areas have cloudy days occasionally or more often.  We are at 38 degrees south, and consider that we have plenty of sunshine, but we still don’t get anything like the potential theoretical capacity, particularly on the boat, but also on our home installation, particularly in winter, when we still try in normal times to sail regularly.  How would the calculations look if you actually only achieved say 50% capacity for say 6 hours per day?  It may not be that bad, but it will give you an idea of the possible range.

Still on charging capacity, the alternator only produces its rated amps at relatively high engine revs, not at a moderate tick over more typical of engine running to charge at anchor, and also only while the battery voltage is low.  As the battery voltage comes up, the charge rate falls off quite quickly.  

None of this says you can’t achieve what you want to do.  But if you start with a realistic estimate of the effect of your assumptions, and have a plan, you will be happier with the outcome.  Have you some space where you could install an additional battery if it became necessary?  Could you fit some extra solar panels to help with charging if it became necessary?  Finally the alternator, how much would be achieved by replacing it with higher capacity depends on how you operate your boat over the day.  There are some good threads in the archives worth searching through, but my impression is that the experts more often than not find replacing the alternator is less beneficial than increasing the solar capacity.

A final thought, wire sizing recommendations are generally based on overhearing the insulation, however, it is important to also cover voltage drop.  It is worth using a size or two bigger than the normal recommendations for the current the inverter will draw, so that you keep the voltage drop to a minimum.  Three percent is often used as a criteria, but I suggest even less is better.

rene460

[sailingabe41ds]

Hi Erik..

I am new here but I owned a Hunter 36 with about 400amphrs and I could run the microwave for short periods of time. For my needs...I found that having a Honda 2000 generator to be very useful for both recharging the batteries in the morning while running the water heater or the microwave or the coffee maker.  You could just upgrade to a Balmar alternator with a smart regulator and add a few xtra batteries if you want to avoid getting a generator. I do not think that 200 amphrs is enough. 

The wiring switches will be  determined by an A/C panel in which you can switch on the water heater or the microwave independent of each other.  It will have to have its own switch on the panel. The rest of the wiring will  depend on the instruction manuals on the inverter/battery charger you buy.  This is something that you may have to hire someone to do if you have a limited knowledge of how these installs should be safely done.


Best,
Abe

[johannes]

The inrush current when a microwave starts may be a concern. In 230V settings you often need type C circuit breakers to handle the current. 

I don't know how people here use their microwaves. We almost never run ours for more than 3 minutes. Typically we heat the leftovers from yesterday's dinner, or a bread roll for 10 seconds. I sometimes miss having one on the boat just for these purposes, but not enough to bother to install one.

[Erik]

Thanks everyone for the comments and advice.  I will let you know how it goes!

[sunrise4]

Erik, one aspect I have not seen mentioned above is voltage drop on your battery as you draw that much current, even if only for minutes, which at some point will shut off your inverter.  I don't have a microwave, but have tested this with a 800 watt hairdryer to see if our inverter (Victron Phoenix 1200VA) actually 'works'.  The voltage drop can be overcome by running the engine at idle (in our case with our battery/alternator setup).

BTW, I really like the Victron products as you can monitor them on your cell phone via bluetooth and don't need extra switches or gauges installed.

Good luck with your project!

Cheers!

[Erik]

Good point, thanks! I also like Victron products and have 2 MPPTs and a battery monitor.

[sunrise4]

Erik, forgot to mention about wiring...

I installed a DPDT switch (line & neutral switched) in front of the GFCI under our chart table, this way all our outlets are either on shore power or inverter power and protected by the GFCI.  I tied all the grounds together at that point. If I recall correctly that GFCI is a 16A 30mA, so sufficient for a microwave.  I also saw on our 419 one of the cupboards above the stove has a removable back panel with a 110V pigtail behind it, but not sure where that is tied into the rest of 110V circuitry.

Cheers!

[Erik]

Thanks, Sunrise4.  When you say you connected all the grounds together, I assume you mean all the AC grounds?  My SO39i has an AC outlet pre-wired specifically for a microwave.  Installing the microwave for use with shorepower should be as simple as finding a microwave that fits, mounting it appropriately and plugging it in.  The inverter would then be step 2.  Although I say I will use the microwave for 5 minutes a day, it will more likely be 5 minutes per month.  But it will make my wife happy!

[sunrise4]

Yes, AC grounds tied together at that point.

[Erik]

Thanks

[zzmeyer]

I don't know how you will be able to cook your Jimmy Dean breakfast sandwiches or Monterrey frozen burritos without a microwave during those long cold watches at night. A microwave consumes a lot of energy for a short period of time. I had an 800 watt microwave and planned on running it for about 50% longer than package instruction. An 800 watt microwave for 1.5 minutes to get a piping hot bean and cheese burrito at 0200 is a small price to pay.

[Erik]

I participated as crew in the ARC from the US to Europe a few years ago. When the sea was running, it was really hard to use the stove. Putting leftovers in the microwave for a couple of minutes was the way to go!  When we got to Portugal and toured all the other boats that made the crossing, I can't recall a single one that didn't have a microwave....

[Erik]

Hi All -

I successfully completed my microwave plus inverter installation.  Here are the details in case anyone with a similar boat wants to do this:

The microwave is made by Danby DMW7700DLDB available on Amazon for about $70. It's a 700 Watt 0.7 cu ft unit.  Note that it delivers 700 watts to the food it heats up. It takes about 1000 - 1100 watts steady (a little more at startup).  This microwave fits nicely behind the stove, and according to my local dealer is the microwave model that Jeanneau has sometimes installed when ordered from the factory. 

There is an outlet in the cabinet to the left that's designed specifically for putting a microwave in this location, so no need to run an extra AC outlet.  I took out the middle shelf, mounted the microwave to the middle shelf with some L-brackets, and placed the middle shelf with microwave onto the bottom shelf, where it fits perfectly.

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The inverter I chose is a Xantrex ProWatt SW 2000. I bought it with the optional Transfer Relay and Remote on/off swich.  For the DC disconnect switch, I purchased a battery switch off the Jeaneau Spares site which matches the other battery switches.  The boat is pre-configured to accept an additional switch (it's the left-most switch in the picture).



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I ran 1/0 battery cables from the battery bank, with a 250 amp ANL fuse.

I mounted the transfer switch behind the electrical panel in the nav station. It takes 2 feeds, one is the switched shore power output that feeds the AC electrical outlets, the other is the inverter.  It feeds either of these to the electrical outlets. If both are present, it favors the shore power.  Hot water heater and battery charger circuits are separate and never powered by the inverter.

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I haven't used it extensively as the boat is now on the hard, but it heated a cup of cold water nicely in 60 seconds, after which point the battery monitor showed 99% instead of 100%.  About 10 minutes later I was back to 100% from solar charging.  I don't plan to use it for cooking or even making hot water... just for reheating things from time to time.

At present I have a smallish 264 amp hour battery bank with 265 watts of solar.  My longer term plan is to increase both of these, but for the moment, it seems sufficient.

[Erik]

Here are a few more pictures of the installation:






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